Duplex stainless steel, duplex stainless steel slab, and duplex stainless steel material

ABSTRACT

One aspect of this duplex stainless steel contains, in mass %, C: 0.03% or less, Si: 0.05% to 1.0%, Mn: 0.1% to 7.0%, P: 0.05% or less, S: 0.0001% to 0.0010%, Ni: 0.5% to 5.0%, Cr: 18.0% to 25.0%, N: 0.10% to 0.30%, Al: 0.05% or less, Ca: 0.0010% to 0.0040%, and Sn: 0.01% to 0.2%, with the remainder being Fe and inevitable impurities, wherein a ratio Ca/O of the amounts of Ca and O is in a range of 0.3 to 1.0, and a pitting index PI shown by formula (1) is in a range of less than 30, 
       PI=Cr+3.3Mo+16N  (1).

TECHNICAL FIELD

The present invention relates to an inexpensive Sn-containing duplexstainless steel. In addition, the present invention relates to aninexpensive duplex stainless steel which contains a combination of Cuand Sn and which is excellent in corrosion resistance. In detail, thepresent invention relates to a duplex stainless steel, a duplexstainless steel slab (a cast steel of a duplex stainless steel), and aduplex stainless steel material which are able to be used in a seawaterdesalination unit, tanks for a transport ship, various types ofcontainers, or the like.

This application is a national stage application of InternationalApplication No. PCT/JP2012/076821, filed on Oct. 17, 2012, which claimspriority to Japanese Patent Application No. 2011-231352 filed on Oct.21, 2011 in Japan, and Japanese Patent Application No. 2011-266351 filedon Dec. 6, 2011 in Japan, the contents of which are incorporated hereinby reference.

BACKGROUND ART

A general-purpose duplex stainless steel contains a large amount of Cr,Mo, Ni, and N and has favorable corrosion resistance. However, as aresult of containing Mo and Ni, which are expensive, the alloy cost ishigh and the manufacturability is not favorable. As a result, the priceof steel material is not cheap and the duplex stainless steel is notwidely used in place of 316 grade stainless steel or 317 grade stainlesssteel. Here, the general-purpose duplex stainless steel referred to inthe present invention indicates duplex stainless steel having thepitting index PI (represented by the following formula which is the sumof the amounts of the alloy elements: PI=Cr+3.3Mo+16N) of approximately30 or more to less than 40 (mass %). From the circumstances describedabove, in such steels, it is considered that there is a need for steelswhere the alloy cost is lower than that in the related art and themanufacturing costs are inexpensive and which have favorable hotmanufacturability while exhibiting the same level of corrosionresistance as the general-purpose duplex stainless steel of the relatedart.

On the other hand, recently, an alloy-saving type duplex stainless steelin which amounts of Cr, Ni, Mo, and the like are reduced has beendeveloped. Here, the alloy-saving type duplex stainless steel indicatesa stainless steel which exhibits a pitting resistance equivalent tothose of SUS 304 and 316L and where the pitting resistance index PI(=Cr+3.3Mo+16N), which is indexed by the amounts of the alloy elements,is approximately in a range of less than 30. In these steels where theamounts of alloy elements which are effective for pitting resistance andacid resistance are reduced, it is difficult to obtain the same level ofcorrosion resistance as that of the general-purpose duplex stainlesssteel. However, it is considered that it is possible to develop improvedsteels by using inexpensive alternative elements.

Various types of duplex stainless steels which contain Sn have beenproposed in the related art. For example, duplex stainless steels aredisclosed which contain 25% or more of Cr and contain 0.01% to 0.1% ofSn as a selected element (refer to Patent Documents 1 and 2 describedbelow). In addition, alloy-saving type duplex stainless steels aredisclosed which contain 1% or less or 0.1% of Sn (refer to PatentDocuments 3 and 4 described below). In the Patent Documents, an objectis to improve the corrosion resistance by means of the amount of Sn;however, the relationship between the hot manufacturability of the steelmaterial and the amount of Sn was not investigated.

In addition, in the Patent Documents described above, the subject is asteel where the amount of N is in a range of 0.2% or less. N is anelement which decreases the hot workability of the stainless steel.Ensuring a desired level of hot workability of a duplex stainless steelwhich contains 0.2% or more of N is more difficult than ensuring adesired level of hot workability of a duplex stainless steel whichcontains less than 0.2% of N. Technical literature which makes adisclosure regarding the hot workability of a duplex stainless steelwhich contains 0.20% or more of N and further contains a combination ofSn and Cu is not to be found.

The present inventors focused on the possibility of improving the acidresistance and the pitting resistance using Sn in an alloy-saving typeduplex stainless steel. Then, the present inventors investigated therelationship between the amount of Sn and the corrosion resistance andthe hot manufacturability. As a result, it was found that it waspossible to improve the corrosion resistance by 0.01% to 0.2% of Snbeing contained. However, it was learned that the hot manufacturabilitydecreased in duplex stainless steels which contained a large amount ofSn. For this reason, the frequency of decreases in the yield of thesteel material will increase and a significant cost increase ispredicted.

In addition, the present inventors focused on the possibility ofimproving the acid resistance and the pitting resistance using Sn and Cuin the general-purpose duplex stainless steel. Then, with regard to theduplex stainless steel where the amounts of Mo and Ni are reduced andwhich contains 0.20% or more of N, the present inventors investigatedthe relationship between the amounts of Sn and Cu, the corrosionresistance, and the hot manufacturability. As a result, it was foundthat it was possible to improve the corrosion resistance by 0.01% to0.2% of Sn and 0.2% to 3.0% of Cu being contained. However, it waslearned that the hot manufacturability decreased in duplex stainlesssteels which contained a large amount of Sn and Cu. For this reason, thefrequency of decreases in the yield of the steel material will increaseand a significant cost increase is predicted.

The present inventors investigated the knowledge of the related artrelating to the manufacturing techniques for Sn-containing duplexstainless hot-rolled steel material of the related art starting withPatent Documents 1 to 4. As a result, it was found that there was littleknowledge with regard to the relationship between the temperature rangewhere hot embrittlement occurs due to Sn which is included in the duplexstainless steel and the amount of Sn and the relationship with theamounts of other elements.

PRIOR ART DOCUMENT Patent Documents

-   Patent Document 1: Japanese Unexamined Patent Application, First    Publication No. H3-158437-   Patent Document 2: Japanese Unexamined Patent Application, First    Publication No. H4-072013-   Patent Document 3: Japanese Unexamined Patent Application, First    Publication No. 2010-222593-   Patent Document 4: PCT International Publication No. W02009-119895-   Patent Document 5: Japanese Unexamined Patent Application, First    Publication No. 2002-69592-   Patent Document 6: Japanese Unexamined Patent Application, First    Publication No. H7-118805

Non-Patent Document

-   Non-Patent Document 1: “Effect of Cu and Ni on Hot Workability of    Hot-rolled Mild Steel” ISIJ, Vol. 37, p. 217 to 223 (1997)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention finds a measure for solving the problems describedabove by clarifying the relationship between the amount of Sn and hotmanufacturability in an alloy-saving type duplex stainless steel. Inaddition, the present invention finds a measure for solving the problemsdescribed above by clarifying the relationship between the amounts of Snand Cu and hot manufacturability in a general-purpose duplex stainlesssteel. Due to this, the object of the present invention is to provide anSn-containing duplex stainless steel, a cast steel of a duplex stainlesssteel, and a duplex stainless steel material which are inexpensive andhave favorable hot manufacturability. Such a duplex stainless steel isexpected to have an excellent balance between corrosion resistance andcost. For this reason, it is considered that the possibility that theduplex stainless steel will be widely used in various fields is high.

In particular, an object of a second aspect (a second embodiment) of theinvention is to develop an inexpensive general-purpose duplex stainlesssteel where the amounts of Ni and Mo, which are expensive elements, arereduced by increasing the amounts of N and Mn and adding a combinationof Cu and Sn.

Means for Solving the Problems

In order to solve the problems described above, for the alloy-savingtype duplex stainless steel which is the subject of the presentinvention, the present inventors prepared melted materials where theamount of Sn and the amounts of Ca, B, rare earth elements (REM), or thelike were changed and performed the following experiments. Here, theamounts of Ca, B, rare earth elements (REM), or the like are said toimprove the hot manufacturability.

Tensile test pieces were collected from cast steels which were cast fromthe melted materials. High temperature tensile test was performed at atemperature of 1200 to 700° C. with respect to the tensile test pieces,and the high temperature ductility was evaluated by measuring thereduction of area (cross-sectional reduction ratio of the fracturesurface). In addition, a hot-rolled steel plate with a plate thicknessof 12 mm was obtained by hot forging and hot rolling and the edgecracking resistance was evaluated. The edge cracking resistance wasevaluated by changing the heating temperature and the rollingtemperature of the hot rolling with respect to a part of the steel, anda correlation of the heating temperature and the rolling temperature ofthe hot rolling with the high temperature ductility was determined.

As disclosed in Patent Documents 5 and 6 described above, generally, induplex stainless steels, it is known that significant edge cracking isgenerated in the hot rolling of the cast steel in most cases where thereduction of area of the cast steel, which is evaluated by hightemperature tensile test, falls below 60%. For this reason, engineers inthis field often subject steels to refining, casting, and hot workingfor the purpose of setting the reduction of area of the cast steel athigh temperatures to be in a range of 60% or more. Here, when thepresent inventors evaluated the high temperature ductility of thealloy-saving type duplex stainless steel (base composition: 21% Cr—2%Ni—3% Mn —0.18% N) cast steel which contains around 0.1% of Sn, it wasclear that all the reductions of area fell below 60% in several meltingexperiments. The evaluation of high temperature ductility was performedas follows. First, a parallel section of a round bar of 8 mmφ was heatedto 1200° C. using a high frequency. Next, the temperature was lowered toa temperature for performing a break test, and tensile rupture wasperformed at a rate of 20 mm/second at this temperature. Then, theshrinkage ratio of the cross section was determined. An example of thedata is shown in FIG. 1. From these results, it was considered thatthere was almost no hope of obtaining an inexpensive alloy-saving typeduplex stainless steel with added Sn in practice.

The present inventors observed an edge cracking length which wasgenerated when a cast steel of an alloy-saving type Sn-containing duplexstainless steel, which was obtained by vacuum melting and casting, wassubjected to hot rolling. As a result, it was found that there rarelyexists a cast steel of an Sn-containing duplex stainless steel in whicha number of edge cracks is small. Hot rolling experiments were performedas follows. First, a cast steel with a thickness of 90 to 44 mm washeated to 1200° C. Next, the thickness of the cast steel was reduced toa thickness of 12 to 6 mm by a plurality of rolling passes. Thefinishing rolling temperature was controlled to be approximately 900° C.Edge cracking was generated on the left and right sides and the maximumlengths on both sides were totaled to obtain the edge cracking length.Even when the edge cracking length of the steel material was looked uponas being related to the minimum value (the minimum value is obtained atapproximately 900° C. in FIG. 1) of the reduction of area of the hightemperature ductility of the cast steel, it was not possible to obtain aclear correlation. However, when the edge cracking length was lookedupon as being related to the reduction of area at 1000° C. as shown inFIG. 2, it was clear that a good correlation is exhibited regardless ofwhether or not Sn is contained. Here, in FIG. 2, the points which areplotted by ∘ (open circles) correspond to the results of Sn-A and Sn-Bof FIG. 1, and the points which are plotted by ♦ (black diamonds) arethe other experiment results (the experiment results examined regardlessof whether or not Sn is contained).

The present inventors performed melting, casting, and rollingexperiments while further changing the amounts of various elements inorder to find the conditions for reliably obtaining a cast steel withlittle edge cracking as described above. Then, the evaluation of thehigh temperature ductility of the cast steel and the evaluation of edgecracking of the steel material after hot rolling were activelyperformed. The first aspect of the present invention where theinexpensive Sn-containing alloy-saving type duplex stainless steel isspecified was completed on the basis of the findings which were obtainedthrough the above experiments.

The requirements of the first aspect of the duplex stainless steel ofthe present invention are shown below.

(1) A duplex stainless steel which includes, in mass %: C: 0.03% orless; Si: 0.05% to 1.0%; Mn: 0.1% to 7.0%; P: 0.05% or less; S: 0.0001%to 0.0010%; Ni: 0.5% to 5.0%; Cr: 18.0% to 25.0%; N: 0.10% to 0.30%; Al:0.05% or less; Ca: 0.0010% to 0.0040%; and Sn: 0.01% to 0.2%, with theremainder being Fe and inevitable impurities, wherein a ratio Ca/O ofthe amounts of Ca and O is in a range of 0.3 to 1.0, and a pitting indexPI shown by formula (1) is in a range of less than 30.

PI=Cr+3.3Mo+16N  (1)

(The chemical symbols in the formula (1) indicate the amounts of theelements).

(2) The duplex stainless steel according to (1), which further includesone or more selected from Mo: 1.5% or less, Cu: 2.0% or less, W: 1.0% orless, and Co: 2.0% or less.(3) The duplex stainless steel according to (1) or (2), which furtherincludes one or more selected from V: 0.05% to 0.5%, Nb: 0.01% to 0.20%,and Ti: 0.003% to 0.05%.(4) The duplex stainless steel according to any one of (1) to (3), whichfurther includes one or more selected from B: 0.0050% or less, Mg:0.0030% or less, and REM: 0.10% or less.

In addition, in order to solve the problems described above, with regardto the general-purpose duplex stainless steel which is the subject ofthe present invention, the present inventors prepared melted materialswhere the amount of Sn, the amounts of Ca, B, rare earth elements (REM),and the like and the amount of Ni were changed and where Co was furtheradded, and they performed the following experiments. Here, it is saidthat the hot manufacturability is improved by containing Ca, B, rareearth elements (REM), and the like.

Tensile test pieces were collected from a cast steel which was cast fromthe melted materials. The tensile test pieces were subjected to hightemperature tensile test at a temperature of 1200 to 700° C., and thehigh temperature ductility was evaluated by measuring the reduction ofarea (cross-sectional reduction ratio of the fracture surface). Inaddition, a hot-rolled steel plate with a plate thickness of 12 mm wasobtained by hot forging and hot rolling, and the edge crackingresistance was evaluated. The edge cracking resistance was evaluated bychanging the heating temperature and the rolling temperature of the hotrolling with respect to a part of the steel, and a correlation of theheating temperature and the rolling temperature of the hot rolling withthe high temperature ductility was determined.

As disclosed in Patent Documents 5 and 6 described above, generally, induplex stainless steels, it is known that significant edge cracking isgenerated in the hot rolling of the cast steel in most cases where thereduction of area of the cast steel, which is evaluated by hightemperature tensile test, falls below 60%. For this reason, engineers inthis field often subject steels to refining, casting, and hot workingfor the purpose of setting the reduction of area of the cast steel athigh temperatures to be in a range of 60% or more. Here, when thepresent inventors evaluated the high temperature ductility of thegeneral-purpose cast steel of a duplex stainless steel (basecomposition: 25% Cr—4% Ni —1.2% Mo—1.5% Cu—0.25% N) which containsaround 0.1% of Sn, it was clear that the minimum values of all thereductions of area fell below 60% in several melting experiments. Theevaluation of high temperature ductility was performed as follows.First, a parallel section of a round bar of 8 mmφ was heated to 1200° C.using a high frequency. Next, the temperature was lowered to atemperature for performing a break test, and tensile rupture wasperformed at a rate of 20 mm/second at this temperature. Then, theshrinkage ratio of the cross section was determined. An example of thedata is shown in FIG. 3. From these results, it was considered thatthere was almost no hope of obtaining an inexpensive general-purposeduplex stainless steel with added Sn in practice.

The present inventors observed an edge cracking length which wasgenerated when a cast steel of a general-purpose duplex stainless steel,which was obtained by vacuum melting and casting, was subjected to hotrolling. As a result, it was discovered that there rarely exists anSn-containing duplex stainless steel material in which a number of edgecracks is small. Hot rolling experiments were performed as follows.First, a cast steel with a thickness of 90 to 44 mm was heated to 1200°C. Next, the thickness of the cast steel was reduced to a thickness of12 to 6 mm by a plurality of rolling passes. The finishing rollingtemperature was controlled to be approximately 900° C. Edge cracking wasgenerated on the left and right sides and the maximum lengths on bothsides were totaled to obtain the edge cracking length. Even when theedge cracking length of the steel material was looked upon as beingrelated to the minimum value (the minimum value is obtained atapproximately 900° C. in FIG. 3) of the reduction of area of the hightemperature ductility of the cast steel, it was not possible to obtain aclear correlation. However, when the edge cracking length was lookedupon as being related to the reduction of area at 1000° C. as shown inFIG. 4, it was clear that a good correlation is exhibited regardless ofwhether or not Sn is contained. Here, in FIG. 4, the points which areplotted by ∘ (open circles) correspond to the results of Sn-A and Sn-Bof FIG. 3, and the points which are plotted by ♦ (black diamonds) arethe other experiment results (the experiment results examined regardlessof whether or not Sn is contained).

The present inventors performed melting, casting, and rollingexperiments while further changing the amounts of various elements inorder to find the conditions for reliably obtaining a steel materialwith little edge cracking as described above. Then, the evaluation ofthe high temperature ductility of the cast steel and the evaluation ofthe edge cracking of the steel material after hot rolling were activelyperformed. The second aspect of the present invention where theinexpensive Sn-containing duplex stainless steel is specified wascompleted on the basis of the findings which were obtained through theabove experiments.

The requirements of the second aspect of the duplex stainless steel ofthe present invention are shown below.

(5) A duplex stainless steel which includes, in mass %: C: 0.03% orless; Si: 0.05% to 1.0%; Mn: 0.1% to 4.0%; P: 0.05% or less; S: 0.0001%to 0.0010%; Cr: 23.0% to 28.0%; Ni: 2.0% to 6.0%; Co: 0% to 1.0%; Cu:0.2% to 3.0%; Sn: 0.01% to 0.2%; N: 0.20% to 0.30%; Al: 0.05% or less;and Ca: 0.0010% to 0.0040%, with the remainder being Fe and inevitableimpurities, wherein Ni+Co is in a range of 2.5% or more and a ratio Ca/Oof the amounts of Ca and O is in a range of 0.3 to 1.0, and PI shown byformula (1) is in a range of 30 or more and less than 40.

PI=Cr+3.3Mo+16N  (1)

(The chemical symbols in the formula (1) indicate the amounts of theelements).

(6) The duplex stainless steel according to (5), which further includeseither one or both of Mo: 2.0% or less, and W: 1.0% or less.(7) The duplex stainless steel according to (5) or (6), which furtherincludes one or more selected from V: 0.05% to 0.5%, Nb: 0.01% to 0.15%,and Ti: 0.003% to 0.05%.(8) The duplex stainless steel according to any one of (5) to (7), whichfurther includes one or more selected from B: 0.0050% or less, Mg:0.0030% or less, and REM: 0.10% or less.

The requirements of one aspect of the cast steel of the duplex stainlesssteel and the duplex stainless steel material of the present inventionare shown below.

(9) A cast steel of a duplex stainless steel which has a compositionaccording to any one of (1) to (8), wherein a fracture reduction of areaat 1000° C. is in a range of 70% or more.(10) A duplex stainless steel material which is manufactured by hotworking the cast steel of the duplex stainless steel according to (9).

Effects of the Invention

According to an aspect of the present invention, it is possible toprovide a duplex stainless steel, a cast steel of a duplex stainlesssteel, and a duplex stainless steel material which have improvedcorrosion resistance compared to a steel used in the related art as thematerial for seawater desalination unit, tanks for a transport ship,various types of containers, or the like in addition to an excellentbalance with cost. For this reason, the aspects of the present inventionmake a significant contribution to industrial development.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram which illustrates the high temperature ductility ofSn-containing and Sn-free duplex stainless steels associated with thefirst aspect of the duplex stainless steel (an alloy-saving type duplexstainless steel).

FIG. 2 is a diagram which shows the relationship between the edgecracking length after hot rolling and the reduction of area at 1000° C.associated with the first aspect of the duplex stainless steel (thealloy-saving type duplex stainless steel).

FIG. 3 is a diagram which illustrates the high temperature ductility ofSn-containing and cast steels of Sn-free duplex stainless steelsassociated with the second aspect of the duplex stainless steel (ageneral-purpose duplex stainless steel).

FIG. 4 is a diagram which shows the relationship between the edgecracking length after hot rolling and the reduction of area at 1000° C.associated with the second aspect of the duplex stainless steel (thegeneral-purpose duplex stainless steel).

EMBODIMENTS OF THE INVENTION First Embodiment

Below, description will be given of the reasons for limiting the firstaspect (the alloy-saving type duplex stainless steel) of the duplexstainless steel of the present invention. Here, the amounts of therespective components are shown in terms of mass %.

Here, in the present embodiment, the cast steel of the stainless steelindicates a steel in a state after casting and before processing such ashot working, forging, or the like is performed, and the stainless steelmaterial indicates a semi-finished product, a hot-rolled steel plate, acold-rolled steel plate, a steel wire, a steel pipe, or the like afterprocessing the cast steel by various methods. In addition, the stainlesssteel indicates general forms for a steel such as a cast steel, a steelmaterial, and the like. The processing described above includes hot andcold processings.

In order to ensure the corrosion resistance of the stainless steel, theamount of C is limited to be in a range of 0.03% or less. When more than0.03% of C is contained, the corrosion resistance and toughness aredegraded due to the generation of Cr carbides during hot rolling.

0.05% or more of Si is added for deoxidation. However, when more than1.0% of Si is added, the toughness is degraded. Therefore, the upperlimit for the amount of Si is limited to 1.0%. The preferable range forthe amount of Si is in a range of 0.2% to 0.7%.

Mn has the effect of improving the toughness by increasing the austenitephase. In addition, since Mn has the effect of decreasing the nitrideprecipitation temperature TN, it is preferable to actively add Mn to thesteel material of the present embodiment. For the toughness of the basematerial and the welding sections, 0.1% or more of M is added. However,when more than 7.0% of Mn is added, the corrosion resistance and thetoughness are degraded. Therefore, the upper limit for the amount of Mnis limited to 7.0%. The amount of Mn is preferably in a range of 1.0% to6.0%, and more preferably in a range of 2.0% to 5.0%.

P is an element which is inevitably mixed in from raw materials and theamount of P is limited to be in a range of 0.05% or less since Pdegrades the hot workability and the toughness. The amount of P ispreferably in a range of 0.03% or less.

S is an element which is inevitably mixed in from the raw materials andthe amount of S is limited to be in a range of 0.0010% or less since Sdegrades the hot workability, the toughness, and the corrosionresistance. In addition, reducing the amount of S to less than 0.0001%increases the costs due to desulfurization refining. For this reason,the amount of S is set to be in a range of 0.0001% to 0.0010%. Theamount of S is preferably in a range of 0.0002% to 0.0006%.

Since Ni stabilizes the austenitic structure and improves the toughnessand the corrosion resistance with respect to various types of acid, 0.5%or more of Ni is contained. By increasing the amount of Ni, it ispossible to decrease the precipitation temperature of nitrides. On theother hand, Ni is an expensive alloy, and from the point of view ofcosts, the amount of Ni is limited to be in a range of 5.0% or less inthe steel of the present embodiment where the subject is an alloy-savingtype duplex stainless steel. The amount of Ni is preferably in a rangeof 1.0% to 4.0%, and more preferably in a range of 1.5% to 3%.

In order to ensure the basic corrosion resistance, 18.0% or more of Cris contained. On the other hand, when more than 25.0% of Cr iscontained, the ferrite phase fraction increases and the toughness andthe corrosion resistance of the welding sections are inhibited. For thisreason, the amount of Cr is set to be in a range of 18.0% or more and25.0% or less. The amount of Cr is preferably in a range of 19.0% to23.0%.

N is an element which is effective for increasing the strength and thecorrosion resistance by being solid-solubilized in the austenite phase.For this reason, 0.10% or more of N is contained. On the other hand, thesolid solubility limit is increased according to the amounts of Cr andMn; however, when more than 0.30% of N is contained in the steel of thepresent embodiment, Cr nitrides are precipitated such that the toughnessand the corrosion resistance are inhibited and the hot manufacturabilityis inhibited. For this reason, the upper limit of the amount of N is setto 0.30%. The amount of N is preferably in a range of 0.10% to 0.25%.

Al is an element which deoxidizes a steel and reduces the oxygen in thesteel according to necessity. For this reason, Al is contained togetherwith 0.05% or more of Si. In an Sn-containing steel, the reduction ofthe oxygen amount is essential in order to ensure the hotmanufacturability, and for this reason, it is necessary that 0.003% ormore of Al be contained according to necessity. On the other hand, Al isan element having comparatively large affinity with N, and when anexcessive amount of Al is added, the toughness of the stainless steel isinhibited due to the generation of AlN. The degree also depends on theamount of N; however, when the amount of Al exceeds 0.05%, the toughnessis greatly decreased. For this reason, the upper limit of the amount ofAl is set to 0.05%. The amount of Al is preferably in a range of 0.04%or less.

Ca is an important element for the hot manufacturability of the steel,and it is necessary that Ca be contained in order to fix S and O in thesteel as inclusions and to improve the hot manufacturability. In thesteel of the present embodiment, 0.0010% or more of Ca is contained forthis purpose. In addition, addition of an excessive amount thereofdecreases the pitting resistance. For this reason, the upper limit ofthe amount of Ca is set to 0.0040%.

Sn is contained in order to improve the corrosion resistance of thesteel of the present embodiment. For this reason, it is necessary thatat least 0.01% of Sn be contained. Furthermore, it is preferable that0.02% or more of Sn be contained. On the other hand, Sn is an elementwhich inhibits the hot manufacturability of the steel, and decreases thehot strength of the interface between the ferrite phase and theaustenite phase, particularly at a temperature of 900° C. or less in thealloy element saving type duplex stainless steel which is the subject ofthe present embodiment. The degree of the decrease depends on theamounts of S, Ca, and O; however, when more than 0.2% of Sn iscontained, it is not possible to prevent the decrease in the hotmanufacturability even by restricting other limits in the presentembodiment. Therefore, the upper limit of the amount of Sn is set to0.2%.

The ratio Ca/O of the amounts of O and Ca is an important componentindex in order to improve the hot manufacturability and the corrosionresistance of the steel of the present embodiment. The lower limit ofCa/O is limited in order to improve the hot manufacturability of theSn-containing steel. The high temperature ductility of the Sn-containingsteel is decreased, particularly at a temperature of 900° C. or less.When the value of Ca/O is in a range of less than 0.3, the hightemperature ductility at 1000° C. is also decreased and the hotmanufacturability is greatly impaired. For this reason, Ca/O is limitedto be in a range of 0.3 or more in the steel of the present embodiment.On the other hand, when an excessive amount of Ca is added and Ca/Oexceeds 1.0, the pitting resistance is impaired. In addition, when theamount of Ca is excessive, the high temperature ductility at atemperature of 1000 to 1100° C. is also impaired. For this reason, theupper limit of Ca/O is set to be in a range of 1.0. Ca/O is preferablyin a range of 0.4 to 0.8.

O is an inevitable impurity and an upper limit thereof is notparticularly set; however, O is an important element which configuresoxides which are the representative of non-metallic inclusions.Composition control of the oxides is extremely important for theimprovement of the hot manufacturability. In addition, surface defectsare caused when coarse cluster-shaped oxides are generated. For thisreason, it is necessary to limit the amount of O so as to be low. In thepresent embodiment, as described above, by setting the ratio of theamount of Ca and the amount of O to be in a range of 0.3 or more, theamount of O is limited. The upper limit of the amount of O is preferablyin a range of 0.005% or less.

In order to incrementally increase the corrosion resistance, one or moreselected from Mo: 1.5% or less, Cu: 2.0% or less, W: 1.0% or less, andCo: 2.0% or less may be contained according to necessity. Descriptionwill be given of the reasons for these limits.

Mo is an element which is extremely effective at incrementallyincreasing the corrosion resistance of the stainless steel, and Mo canbe contained according to necessity. In order to improve the corrosionresistance, it is preferable that 0.2% or more of Mo be contained. Onthe other hand, Mo is an element which promotes precipitation ofintermetallic compounds, and the upper limit of the amount of Mo is setto 1.5% from the point of view of suppressing precipitation in the steelof the present embodiment during hot rolling.

Cu is an element which incrementally increase the corrosion resistanceof the stainless steel with respect to acid, and Cu has an effect ofimproving the toughness; and therefore, it is recommended that 0.3% ormore be contained according to necessity. When more than 2.0% of Cu iscontained, the amount of Cu exceeds the solid solubility; and thereby,ε-Cu is precipitated during hot rolling to cause embrittlement. For thisreason, the upper limit of the amount of Cu is set to 2.0%. In a casewhere Cu is contained, the amount is preferably in a range of 0.3% to1.5%.

W is an element which incrementally increases the corrosion resistanceof the stainless steel in the same manner as Mo, and W can be addedaccording to necessity. For the purpose of increasing the corrosionresistance in the steel of the present embodiment, the upper limit ofthe amount of W is set to 1.0%. The amount of W is preferably in a rangeof 0.05% to 0.5%.

Co is an element which is effective for increasing the toughness and thecorrosion resistance of the steel and which is selectively added. Theamount of Co is preferably in a range of 0.03% or more. When more than2.0% of Co is contained, an effect which is commensurate with the costis not exhibited as Co is an expensive element. For this reason, theupper limit of the amount of Co is set to 2.0%. In a case where Co isadded, the amount is preferably in a range of 0.03% to 1.0%.

Furthermore, one or more selected from V: 0.05% to 0.5%, Nb: 0.01% to0.20%, and Ti: 0.003% to 0.05% may be contained. These are elementswhich are more likely to generate nitrides rather than Cr. V, Nb, and Tican be added according to necessity, and there is a tendency for thecorrosion resistance to be improved in cases where these are containedin trace amounts.

Nitrides and carbides which are formed by V are generated in the hotworking and the cooling process of the steel material, and these havethe effect of increasing the corrosion resistance. The reasons thereforare not sufficiently confirmed; however, it is considered that there isa probability of suppressing the generation speed of the chromiumnitrides at a temperature of 700° C. or less. 0.05% or more of V iscontained in order to improve the corrosion resistance. When more than0.5% of V is contained, coarse V carbonitrides are generated, andtoughness is degraded. Therefore, the upper limit of the amount of V islimited to 0.5%. In a case where V is added, the amount is preferably ina range of 0.1% to 0.3%.

Nitrides and carbides which are formed by Nb are generated in the hotworking and the cooling process of the steel material, and these havethe effect of increasing the corrosion resistance. The reasons thereforare not sufficiently confirmed; however, it is considered that there isa probability of suppressing the generation speed of the chromiumnitrides at a temperature of 700° C. or less. 0.01% or more of Nb iscontained in order to improve the corrosion resistance. On the otherhand, in the case where an excessive amount of Nb is added, Nb isprecipitated as non-solid-solubilized precipitates during heating beforethe hot rolling; and thereby, the toughness is inhibited. For thisreason, the upper limit of the amount of Nb is set to 0.20%. In a casewhere Nb is added, the range of the amount is preferably in a range of0.03% to 0.10%.

Ti is an element which forms oxides, nitrides, and sulfides in verysmall amounts and Ti refines crystal grains in the solidified structureand the structure heated at a high temperature of the steel. Inaddition, in the same manner as V and Nb, Ti also has the property ofreplacing a part of the chromium in the chromium nitrides. With anamount of Ti of 0.003% or more, Ti precipitates are formed. On the otherhand, when more than 0.05% of Ti is contained in the duplex stainlesssteel, the toughness of the steel is impaired due to the generation ofcoarse TiN. For this reason, the upper limit of the amount of Ti is setto 0.05%. A suitable amount of Ti is in a range of 0.005% to 0.020%.

Furthermore, one or more selected from B: 0.0050% or less, Mg: 0.0030%or less, and REM: 0.10% or less may be contained. In order to achievefurther improvement of the hot workability, the B, Mg, and REM to becontained according to necessity are limited as follows.

B, Mg, and REM are all elements which improve the hot workability of thesteel, and one or more thereof is added for this purpose. The additionof an excessive amount of any one of B, Mg, and REM has the oppositeeffect of decreasing the hot workability and the toughness. For thisreason, the upper limits of the above amounts are set as follows. Theupper limit of the amount of B is 0.0050%. The upper limit of the amountof Mg is 0.0030%. The upper limit of the amount of REM is 0.10%.Preferable amounts of respective elements are B: 0.0005% to 0.0030%, Mg:0.0001% to 0.0015%, and REM 0.005% to 0.05%. Here, REM is the sum of theamounts of lanthanoid rare earth elements such as Ce, La, and the like.

By having the characteristics of the duplex stainless steel of thepresent embodiment described above, it is possible to greatly improvethe hot manufacturability of the alloy-saving duplex stainless steelwhich contains Sn.

In the cast steel stage, a fracture reduction of area at 1000° C. is ina range of 70% or more. In addition, by subjecting the cast steel to theprocesses which include the hot working, it is possible to obtain aduplex stainless steel material with a high yield and few surfacedefects.

Second Embodiment

Below, description will be given of the reasons for the limits of thesecond aspect (a general-purpose duplex stainless steel) of the duplexstainless steel of the present invention. Here, the amounts of therespective components are shown in terms of mass %.

Here, in the present embodiment, the cast steel of the stainless steelindicates a steel in a state after casting and before processing such ashot working, forging, or the like is performed, and the stainless steelmaterial indicates a semi-finished product, a hot-rolled steel plate, acold-rolled steel plate, a steel wire, a steel pipe, or the like afterprocessing the cast steel by various methods. In addition, the stainlesssteel indicates the general forms for a steel such as a cast steel, asteel material, and the like. The processing described above includeshot and cold processings.

In order to ensure the corrosion resistance of the stainless steel, theamount of C is limited to be in a range of 0.03% or less. When more than0.03% of C is contained, the corrosion resistance and toughness aredegraded due to the generation of Cr carbides during hot rolling.

0.05% or more of Si is added for deoxidation. However, when more than1.0% of Si is added, the toughness is degraded. Therefore, the upperlimit for the amount of Si is limited to 1.0%. The preferable range forthe amount of Si is in a range of 0.2% to 0.7%.

Mn has the effect of improving the toughness by increasing the austenitephase. In addition, since Mn has the effect of suppressing theprecipitation of nitrides, it is preferable to actively add Mn to thesteel material of the present embodiment. For the toughness of the basematerial and the welding sections, 0.1% or more of Mn is added. However,when more than 4.0% of Mn is added, the corrosion resistance and thetoughness are degraded. Therefore, the upper limit for the amount of Mnis limited to 4.0%. The amount of Mn is preferably in a range of 1.0% to3.5%, and more preferably in a range of 2.0% to 3.0%.

P is an element which is inevitably mixed in from raw materials and theamount of P is limited to be in a range of 0.05% or less since Pdegrades the hot workability and the toughness. The amount of P ispreferably in a range of 0.03% or less.

S is an element which is inevitably mixed in from the raw materials andthe amount of S is limited to in a range of 0.0010% or less since Sdegrades the hot workability, the toughness, and the corrosionresistance. In addition, reducing the amount of S to less than 0.0001%increases the costs due to desulfurization refining. For this reason,the amount of S is set to be in a range of 0.0001% to 0.0010%. Theamount of S is preferably in a range of 0.0002% to 0.0006%.

23.0% or more of Cr is contained in order to ensure basic corrosionresistance. On the other hand, when more than 28.0% of Cr is contained,the ferrite phase fraction increases and the toughness and the corrosionresistance of the welding sections are inhibited. For this reason, theamount of Cr is set to be in a range of 23.0% or more to 28.0% or less.The amount of Cr is preferably in a range of 24.0% to 27.5%.

Ni stabilizes the austenitic structure and improves the toughness andthe corrosion resistance with respect to various types of acid.Furthermore, Ni suppresses a decrease in hot workability due to theaddition of Sn and Cu. For this reason, 2.0% or more of Ni is contained.By increasing the amount of Ni, it is possible to decrease the nitrideprecipitation temperature. On the other hand, since Ni is an expensivealloy, the amount of Ni is limited to be in a range of 6.0% or less. Theamount of Ni is preferably in a range of 2.5% to 5.5%, and morepreferably in a range of 3.0% to 5.0%.

Co is an element which is effective for increasing the toughness and thecorrosion resistance of the steel and which suppresses a decrease in thehot workability due to the addition of Sn and Cu, and it is desirablethat Co be contained together with Ni. In addition, in a case where Cois added, it is preferable that 0.1% or more of Co be contained. Whenmore than 1.0% of Co is contained, an effect which is commensurate withthe cost is not exhibited as Co is an expensive element. For thisreason, the upper limit of the amount of Co is set to 1.0%. In a casewhere Co is added, the amount is preferably in a range of 0.1% to 0.5%.

It is known from Non-Patent Document 1 that Ni increases the solidsolubility of Cu and has an effect of suppressing the generation of aliquid phase having a low melting point due to the addition of Cu andSn. In addition, Co is an element which belongs to the same group as Ni.For this reason, it is considered that the decrease in the hotworkability due to Cu and Sn is suppressed by increasing the sum of theamounts of Ni and Co. The present inventors learned that the edgecracking of the steel material increases in the case where the totalamount of Ni and Co is in a range of less than 2.5% when the hotworkability of the steel which is the subject of the present embodimentis arranged on the sum of the amounts of Ni and Co. For this reason, therange of Ni+Co is set to be in a range of 2.5% or more.

Cu is an element which increases the corrosion resistance of thestainless steel with respect to acid and has an effect of improving thetoughness. In the present embodiment, in order to increase the corrosionresistance, 0.2% or more of Cu is contained together with 0.01% or moreof Sn. When more than 3.0% of Cu is contained, the amount of Cu exceedsthe solid solubility; and thereby, ε-Cu is precipitated during hotrolling to cause embrittlement. For this reason, the upper limit of theamount of Cu is set to 3.0%. In the case where Cu is contained, theamount is preferably in a range of 0.5% to 2.0%.

Sn is contained in order to improve the corrosion resistance of thesteel of the present embodiment. For this reason, it is necessary thatat least 0.01% of Sn be contained. Furthermore, it is preferable that0.02% or more of Sn be contained. On the other hand, Sn is an elementwhich inhibits the hot manufacturability of the steel, and decreases thehot strength of the interface between the ferrite phase and theaustenite phase, particularly at a temperature of 900° C. or less in thealloy element saving type duplex stainless steel which is the subject ofthe present embodiment. The degree of the decrease depends on theamounts of S, Ca, and O; however, when more than 0.2% of Sn iscontained, it is not possible to prevent the decrease in the hotmanufacturability even by restricting other limits in the presentembodiment. Therefore, the upper limit of the amount of Sn is set to0.2%.

N is an element which is effective for increasing the strength and thecorrosion resistance by being solid-solubilized in the austenite phase.For this reason, 0.20% or more of N is contained. Since it is possibleto decrease the amount of Ni by increasing the amount of N, N is anelement which it is desirable to actively add. On the other hand, it isnecessary to limit the upper limit of the amount of N to be within thesolubility limit of N. The solubility limit of N is increased accordingto the amounts of Cr and Mn. When more than 0.30% of N is contained inthe steel of the present embodiment, Cr nitrides are precipitated suchthat the toughness and the corrosion resistance are inhibited and thehot manufacturability is inhibited. For this reason, the upper limit ofthe amount of N is set to 0.30%. The amount of N is preferably in arange of 0.20% to 0.28%.

Al is an element which deoxidizes a steel and Al is contained togetherwith 0.05% or more of Si in order to reduce the oxygen in the steelaccording to necessity. In an Sn-containing steel, the reduction of theoxygen amount is essential in order to ensure the hot manufacturability,and for this reason, it is necessary that 0.003% or more of Al becontained according to necessity. On the other hand, Al is an elementhaving comparatively large affinity with N, and when an excessive amountof Al is added, the toughness of the stainless steel is inhibited due tothe generation of AlN. The degree also depends on the amount of N;however, when the amount of Al exceeds 0.05%, the toughness is greatlydecreased. For this reason, the upper limit of the amount of Al is setto 0.05%. The amount of Al is preferably in a range of 0.04% or less.

Ca is an important element for the hot manufacturability of the steel,and it is necessary that Ca be contained in order to fix the S and O inthe steel as inclusions and to improve the hot manufacturability. In thesteel of the present embodiment, 0.0010% or more of Ca is contained forthis purpose. In addition, addition of an excessive amount thereofdecreases the pitting resistance. For this reason, the upper limit ofthe amount of Ca was set to 0.0040%.

The ratio Ca/O of the amounts of O and Ca is an important componentindex in order to improve the hot manufacturability and the corrosionresistance of the steel of the present embodiment. The lower limit ofCa/O is limited in order to improve the hot manufacturability of theSn-containing steel. The high temperature ductility of the Sn-containingsteel is decreased, particularly at a temperature of 900° C. or less.When the value of Ca/O is in a range of less than 0.3, the hightemperature ductility at 1000° C. is also decreased and the hotmanufacturability is greatly impaired. For this reason, in the steel ofthe present embodiment, Ca/O is limited to be in a range of 0.3 or more.On the other hand, when an excessive amount of Ca is added and Ca/Oexceeds 1.0, the pitting resistance is impaired. In addition, when theamount of Ca is excessive, the high temperature ductility at atemperature of 1000 to 1100° C. is also impaired. For this reason, theupper limit of Ca/O is set to be in a range of 1.0. Ca/O is preferablyin a range of 0.4 to 0.8.

O is an inevitable impurity and an upper limit thereof is notparticularly set; however, O is an important element which configuresoxides which are representatives of non-metallic inclusions. Compositioncontrol of the oxides is extremely important for the improvement of thehot manufacturability. In addition, surface defects are caused whencoarse cluster-shaped oxides are generated. For this reason, it isnecessary to limit the amount of O so as to be low. In the presentembodiment, as described above, by setting the ratio of the amount of Caand the amount of O to be in a range of 0.3 or more, the amount of O islimited. The upper limit of the amount of O is preferably in a range of0.005% or less.

Furthermore, either one or both of Mo: 2.0% or less, and W: 1.0% or lessmay be contained. These are elements which incrementally increase thecorrosion resistance. Description will be given of the reasons for theselimits.

Mo is an element which is extremely effective at incrementallyincreasing the corrosion resistance of the stainless steel, and Mo canbe contained according to necessity. In order to improve the corrosionresistance, it is preferable that 0.2% or more of Mo be contained. Onthe other hand, Mo is an expensive element, and from the point of viewof suppressing the cost of the alloy in the steel of the presentembodiment, the upper limit of the amount of Mo is set to 2.0%.

W is an element which incrementally increases the corrosion resistanceof the stainless steel in the same manner as Mo, and it is possible toadd W according to necessity. For the purpose of increasing thecorrosion resistance in the steel of the present embodiment, the upperlimit of the amount of W is set to 1.0%. The amount of W is preferablyin a range of 0.1% to 0.8%.

Furthermore, one or more selected from V: 0.05% to 0.5%, Nb: 0.01% to0.15%, and Ti: 0.003% to 0.05% may be contained. These are elementswhich are more likely to generate nitrides rather than Cr. It ispossible to add any of V, Nb, and Ti according to necessity, and thereis a tendency for the corrosion resistance to be improved in cases wherethese are contained in trace amounts.

Nitrides and carbides which are formed by V are generated in the hotworking and the cooling process of the steel material, and these havethe effect of increasing the corrosion resistance. The reasons thereforare not sufficiently confirmed; however, it is considered that there isa probability of suppressing the generation speed of the chromiumnitrides at a temperature of 700° C. or less. It is desirable that 0.05%or more of V be contained in order to improve the corrosion resistance.When more than 0.5% of V is contained, coarse V carbonitrides aregenerated and the toughness is degraded. Therefore, the upper limit ofthe amount of V is limited to 0.5%. In a case where V is added, theamount is preferably in a range of 0.1% to 0.3%.

Nitrides and carbides which are formed of Nb are generated in the hotworking and the cooling process of the steel material, and these havethe effect of increasing the corrosion resistance. The reasons thereforare not sufficiently confirmed; however, it is considered that there isa probability of suppressing the generation speed of the chromiumnitrides at a temperature of 700° C. or less. It is desirable that 0.01%or more of Nb be contained in order to improve the corrosion resistance.On the other hand, in the case where an excessive amount of Nb is added,Nb is precipitated as non-solid-solubilized precipitates during heatingbefore the hot rolling; and thereby, the toughness is inhibited. Forthis reason, the upper limit of the amount of Nb is set to 0.15%. In acase where Nb is added, the range of the amount is preferably in a rangeof 0.03% to 0.10%.

Ti is an element which forms oxides, nitrides, and sulfides in verysmall amounts and Ti refines crystal grains in the solidified structureand the structure heated at a high temperature of the steel. Inaddition, in the same manner as V and Nb, Ti also has the property ofreplacing a part of the chromium in the chromium nitrides. With anamount of Ti of 0.003% or more, Ti precipitates are formed. On the otherhand, when more than 0.05% of Ti is contained in the duplex stainlesssteel, the toughness of the steel is impaired due to the generation ofcoarse TiN. For this reason, the upper limit of the amount of Ti is setto 0.05%. A suitable amount of Ti is in a range of 0.005% to 0.020%.

Furthermore, one or more selected from B: 0.0050% or less, Mg: 0.0030%or less, and REM: 0.10% or less may be contained. In order to achievefurther improvement of the hot workability, the B, Mg, and REM to becontained according to necessity are limited as follows.

B, Mg, and REM are all elements which improve the hot workability of thesteel, and it is desirable that one or more be added for this purpose.The addition of an excessive amount of any of B, Mg, and REM has theopposite effect of decreasing the hot workability and the toughness. Forthis reason, the upper limits of the above amounts are set as follows.The upper limit of the amount of B is 0.0050%. The upper limit of theamount of Mg is 0.0030%. The upper limit of the amount of REM is 0.10%.Preferable amounts of respective elements are B: 0.0005% to 0.0030%, Mg:0.0001% to 0.0015%, and REM 0.005% to 0.05%. Here, REM is the sum of theamounts of lanthanoid rare earth elements such as Ce, La, and the like.

Above, by having the characteristics of the duplex stainless steel ofthe present embodiment described above, it is possible to greatlyimprove the hot manufacturability of the general-purpose duplexstainless steel which contains Sn.

In the cast steel stage, a fracture reduction of area at 1000° C. is ina range of 70% or more. In addition, by subjecting the cast steel to theprocesses which include the hot working, it is possible to obtain aduplex stainless steel material with a high yield and few surfacedefects.

EXAMPLES Example 1

Description will be given of examples of the alloy-saving type duplexstainless steel below. The chemical compositions of test steels areshown in Tables 1 to 4. Here, the remainder other than the componentswhich are described in Table 1 is Fe and inevitable impurity elements.In addition, for the components which are shown in Tables 1 to 4,portions where the amounts are not described show the impurity levels.REM indicates lanthanoid rare earth elements, and the amount of REMshows the total of these elements. The numbers which are underlined inthe tables indicate values outside of the ranges which are defined inthe first embodiment.

TABLE 1 Steel No. C Si Mn P S Ni Cr N Al Ca Sn Other O PI Ca/O 1-1Invention Examples 0.015 0.39 3.21 0.022 0.0005 2.15 20.9 0.173 0.0150.0013 0.05 0.0038 23.7 0.34 1-2 0.020 0.34 3.01 0.024 0.0004 2.08 21.00.165 0.042 0.0022 0.09 0.0024 23.6 0.92 1-3 0.018 0.42 4.93 0.0210.0006 2.13 20.9 0.186 0.025 0.0019 0.06 0.0032 23.9 0.59 1-4 0.018 0.353.02 0.023 0.0007 2.35 20.8 0.178 0.023 0.0022 0.13 Mo: 0.32 0.0030 24.70.73 1-5 0.018 0.35 3.05 0.023 0.0007 2.35 20.9 0.168 0.013 0.0028 0.02Cu: 1.05 0.0048 23.6 0.58 1-6 0.018 0.35 3.02 0.024 0.0007 2.35 20.80.181 0.015 0.0018 0.07 W: 0.25 0.0039 23.7 0.46 1-7 0.018 0.35 3.050.023 0.0007 2.35 20.9 0.176 0.012 0.0019 0.03 Co: 0.23 0.0048 23.7 0.401-8 0.021 0.42 2.56 0.031 0.0005 1.53 18.5 0.125 0.043 0.0013 0.05 Mo:1.22, 0.0026 24.5 0.50 Cu: 0.95 1-9 0.021 0.42 2.54 0.031 0.0005 1.4218.5 0.132 0.047 0.0012 0.05 Mo: 1.38, 0.0024 25.2 0.50 Cu: 1.03, Co:0.02 1-10 0.021 0.42 2.53 0.031 0.0005 1.44 18.5 0.115 0.049 0.0015 0.05Mo: 0.12, 0.0028 20.7 0.54 Cu: 1.23, W: 0.23 1-11 0.025 0.64 4.89 0.0260.0006 1.52 21.3 0.215 0.023 0.0023 0.06 V: 0.12 0.0034 24.7 0.68 1-120.025 0.64 5.12 0.026 0.0006 1.52 21.5 0.205 0.021 0.0015 0.06 Nb: 0.0520.0034 24.8 0.44 1-13 0.025 0.64 4.96 0.026 0.0006 1.51 21.7 0.218 0.0150.0018 0.06 Ti: 0.012 0.0038 25.2 0.47 1-14 0.025 0.64 5.32 0.026 0.00061.53 21.5 0.232 0.019 0.0022 0.06 V: 0.11, 0.0036 25.2 0.61 Nb: 0.0351-15 0.028 0.56 1.74 0.023 0.0006 4.53 23.4 0.106 0.035 0.0023 0.17 Mo:0.34, 0.0027 26.2 0.85 V: 0.35, Ti: 0.032

TABLE 2 Steel No. C Si Mn P S Ni Cr N Al Ca Sn Other O PI Ca/O 1-16Invention Examples 0.014 0.45 2.95 0.015 0.0003 1.95 20.7 0.175 0.0230.0013 0.06 Mo: 0.35, 0.0026 24.7 0.50 Cu: 1.04, V: 0.12, Ti: 0.007 1-170.007 0.44 2.98 0.014 0.0003 1.97 20.7 0.175 0.012 0.0036 0.07 W: 0.35,0.0046 23.5 0.78 Co: 0.03, V: 0.11, Ti: 0.006 1-18 0.005 0.46 2.96 0.0130.0003 1.96 20.6 0.173 0.025 0.0021 0.08 Mo: 0.28, 0.0027 24.3 0.78 Cu:1.05, V: 0.14, Nb: 0.048, Ti: 0.011 1-19 0.022 0.15 4.03 0.033 0.00022.03 21.3 0.155 0.023 0.0011 0.10 B: 0.0026 0.0033 23.8 0.33 1-20 0.0230.14 3.26 0.036 0.0010 2.00 21.2 0.165 0.023 0.0031 0.10 Mg: 0.00120.0032 23.8 0.97 1-21 0.024 0.16 3.33 0.023 0.0009 2.04 20.9 0.166 0.0220.0023 0.10 REM: 0.065 0.0034 23.6 0.68 1-22 0.023 0.13 3.12 0.0210.0008 2.03 20.0 0.164 0.024 0.0022 0.10 B: 0.0032, 0.0035 22.6 0.63 Mg:0.0006 1-23 0.021 0.07 2.86 0.019 0.0001 2.05 21.1 0.175 0.021 0.00160.10 B: 0.0023, 0.0023 23.9 0.70 REM: 0.032 1-24 0.022 0.12 2.75 0.0160.0002 2.01 20.9 0.177 0.023 0.0024 0.05 Mo: 0.56, 0.0036 25.6 0.67 Cu:1.45, B: 0.0028

TABLE 3 Steel No. C Si Mn P S Ni Cr N Al Ca Sn Other O PI Ca/O 1-25Invention Examples 0.026 0.76 2.89 0.018 0.0005 2.45 20.8 0.172 0.0220.0028 0.05 Mo: 0.38, 0.0032 24.8 0.88 Cu: 1.06, Co: 0.04, V: 0.13, Ti:0.006, B: 0.0024 1-26 0.024 0.78 3.01 0.015 0.0003 2.56 21.9 0.179 0.0210.0023 0.05 Mo: 0.35, 0.0033 25.9 0.70 Cu: 1.01, W: 0.12, Co: 0.03, V:0.16, Nb: 0.015, Ti: 0.004, B: 0.0016, Mg: 0.0003 1-27 0.016 0.43 6.530.021 0.0004 0.75 18.3 0.182 0.016 0.0016 0.04 Mo: 1.35, 0.0034 25.70.47 Cu: 1.23 1-28 0.024 0.37 2.43 0.023 0.0006 4.58 24.4 0.245 0.0230.0019 0.06 V: 0.13 0.0036 28.3 0.53 1-29 0.013 0.42 3.15 0.022 0.00044.13 24.5 0.235 0.016 0.0022 0.07 0.0046 28.3 0.48 1-30 0.025 0.36 0.230.012 0.0003 3.02 21.1 0.165 0.005 0.0023 0.04 Co: 1.52 0.0047 23.7 0.491-31 0.018 0.26 0.85 0.031 0.0002 4.23 21.3 0.201 0.0021 0.08 W: 0.750.0052 24.5 0.40 1-32 0.023 0.32 2.45 0.024 0.0005 3.24 18.2 0.112 0.0030.0016 0.12 Mo: 1.43 0.0042 24.7 0.38 1-33 0.019 0.39 0.31 0.021 0.00061.68 21.3 0.164 0.013 0.0014 0.06 Cu: 1.83 0.0038 23.9 0.37

TABLE 4 Steel No. C Si Mn P S Ni Cr N Al Ca Sn Other O PI Ca/O 1-AComparative 0.016 0.38 2.96 0.022 0.0006 1.96 20.9 0.174 0.026 0.00060.08 0.0036 23.7 0.17 1-B Examples 0.016 0.38 2.98 0.022 0.0006 1.9620.9 0.174 0.0012 0.08 0.0052 23.7 0.23 1-C 0.015 0.39 2.96 0.023 0.00061.98 21.0 0.172 0.023 0.0016 <0.01  0.0032 23.8 0.50 1-D 0.016 0.38 2.980.022 0.0006 1.97 21.0 0.172 0.023 0.0018 0.26 0.0032 23.8 0.56 1-E0.021 0.42 3.12 0.023 0.0005 2.02 21.1 0.175 0.021 0.0045 0.08 0.002123.9 2.14 1-F 0.043 0.54 2.86 0.025 0.0006 2.01 21.0 0.182 0.017 0.00200.08 0.0036 23.9 0.56 1-G 0.025 1.54 3.13 0.029 0.0006 2.00 21.0 0.1830.017 0.0020 0.07 0.0036 23.9 0.56 1-H 0.024 0.39 7.85 0.028 0.0006 2.0321.0 0.175 0.017 0.0020 0.07 0.0032 23.8 0.63 1-I 0.023 0.46 3.24 0.0650.0005 2.00 21.0 0.186 0.018 0.0020 0.06 0.0033 24.0 0.61 1-J 0.026 0.483.16 0.022 0.0012 1.99 21.0 0.165 0.019 0.0018 0.07 0.0041 23.6 0.44 1-K0.025 0.42 3.08 0.031 0.0006 0.32 21.0 0.159 0.017 0.0007 0.07 0.003523.5 0.20 1-L 0.024 0.41 2.56 0.022 0.0007 2.23 17.2 0.184 0.016 0.00180.12 0.0038 20.1 0.47 1-M 0.025 0.43 2.66 0.023 0.0006 1.85 20.9 0.3300.015 0.0018 0.08 0.0033 26.2 0.55 1-N 0.016 0.44 2.36 0.021 0.0007 1.9621.0 0.174 0.021 0.0019 0.07 V: 0.63 0.0035 23.8 0.54 1-O 0.017 0.422.86 0.025 0.0007 1.94 21.0 0.175 0.019 0.0019 0.07 Nb: 0.24 0.0036 23.80.53 1-P 0.016 0.38 2.94 0.024 0.0007 1.95 21.0 0.173 0.023 0.0019 0.07Ti: 0.062 0.0034 23.8 0.56 1-Q 0.018 0.39 3.11 0.024 0.0007 1.93 21.10.172 0.022 0.0019 0.07 B: 0.0076 0.0036 23.9 0.53 1-R 0.015 0.41 3.130.023 0.0007 1.92 20.9 0.170 0.021 0.0019 0.07 Mg: 0.0041 0.0037 23.60.51 1-S 0.015 0.42 3.06 0.022 0.0007 1.94 21.0 0.169 0.020 0.0019 0.30REM: 0.150 0.0042 23.7 0.45 1-T 0.023 0.38 2.98 0.024 0.0006 2.15 21.30.165 0.072 0.0022 0.05 0.0018 23.9 1.22 1-U 0.023 0.39 2.99 0.0230.0006 2.18 21.1 0.078 0.024 0.0021 0.06 0.0036 22.3 0.58

For all the steels, firstly, a cast steel with a thickness of 100 mm wasprepared, and the fracture reduction of area was evaluated. Theevaluation was performed as follows. First, a parallel section of around bar of 8 mmφ was heated to 1200° C. using a high frequency. Next,the temperature was lowered to a temperature (1000° C.) at which a breaktest was performed. Tensile rupture was performed at a speed of 20mm/second at this temperature, and the shrinkage of the cross sectionwas measured. Steels where the fracture reduction of area was in a rangeof 70% or more were evaluated as A (good), steels where the reduction ofarea was in a range of 60% or more to less than 70% were evaluated as B(fair), steels where the reduction of area was in a range of less than60% were evaluated as C (bad), and the results are given in Tables 5 and6.

The cast steel was subjected to hot forging to obtain a semi-finishedproduct with a thickness of 60 mm, and this semi-finished product wasused as a hot-rolled material. The semi-finished product was heated to apredetermined temperature of 1150 to 1250° C., and then the hot rollingwas performed using a two stage rolling machine in a laboratory underthe following conditions. First, reduction was repeatedly performed soas to adjust the plate thickness to be 25 mm. Then, finishing rollingwas performed from 1000° C., and the final finishing rolling was carriedout at 900° C. This rolling was performed such that the final platethickness became 12 mm and the plate width became 120 mm to obtain ahot-rolled steel plate. The maximum lengths of the edge crackings whichwere generated in the left and right edge sections of the obtainedhot-rolled steel plate were measured, and the sum of the maximum lengthsof the edge crackings in the left and right edge sections wasdetermined. Steels where the sum of the edge crackings was in a range ofless than 5 mm were evaluated as A (good), steels where the sum of theedge crackings was in a range of 5 to 10 mm were evaluated as B (fair),steels where the sum of the edge crackings exceeds 10 mm were evaluatedas C (bad), and the results are given in Tables 5 and 6.

Furthermore, the steel plates were subjected to a solutionizing heattreatment in the following manner. The steel plate was inserted into aheat treatment furnace at 1000° C. and heated for approximately 5minutes. Next, the steel plate was taken out, and then was subjected towater cooling to room temperature.

The corrosion resistance of the steel plate was evaluated by thecorrosion rate in sulfuric acid.

The corrosion rate in the sulfuric acid was measured as follows. Testpieces of 3 mm thick×25 mm wide×25 mm long were subjected to animmersion test for 6 hours in boiling 5% sulfuric acid. The weightbefore and after immersion was measured, and the rate of decrease inweight was calculated. Steels where the corrosion rate in the sulfuricacid was in a range of less than 0.3 g/m² per hour were evaluated as A(good), steels where the corrosion rate in the sulfuric acid was in arange of 0.3 to 1 g/m² per hour were evaluated as B (fair), steels wherethe corrosion rate in the sulfuric acid was in a range of 1 g/m² perhour or more were evaluated as C (bad), and the evaluation results aregiven in Tables 5 and 6.

The impact characteristics were measured using Charpy test pieces whichwere taken a long in the width direction. The test pieces were preparedby processing 2 mm V notches at full size in the rolling direction.Testing was carried out at −20° C. using two test pieces for each of thesteels, and the impact characteristics were evaluated by the averagevalues of the obtained impact values. Steels where the impact value wasin a range of more than 100 J/cm² were evaluated as A (good), steelswhere the impact value was in a range of 50 to 100 J/cm² were evaluatedas B (fair), steels where the impact value was less than 50 J/cm² wereevaluated as C (bad), and the evaluation results are given in Tables 5and 6.

TABLE 5 Edge Sulfuric Impact Cracking Acid Character- ReductionResistance Resistance istics Steel of Area of of Steel of Steel of SteelNo. Cast Steel Material Material Material Invention 1-1 A A A A Examples1-2 A A A A 1-3 A A A A 1-4 A A A A 1-5 A A A A 1-6 A A A A 1-7 A A A A1-8 A A A A 1-9 A A A A 1-10 A A A A 1-11 A A A A 1-12 A A A A 1-13 A AA A 1-14 A A A A 1-15 A A A A 1-16 A A A A 1-17 A A A A 1-18 A A A A1-19 A A A A 1-20 A A A A 1-21 A A A A 1-22 A A A A 1-23 A A A A 1-24 AA A A 1-25 A A A A 1-26 A A A A 1-27 A A A A 1-28 A A A A 1-29 A A A A1-30 A A A A 1-31 A A A A 1-32 A A A A 1-33 A A A A

TABLE 6 Edge Sulfuric Impact Cracking Acid Character- ReductionResistance Resistance istics Steel of Area of of Steel of Steel of SteelNo. Cast Steel Material Material Material Comparative 1-A C C A AExamples 1-B B C A A 1-C A A B A 1-D C C A B 1-E C C A A 1-F B B A B 1-GA A A B 1-H B C B A 1-I C C A B 1-J C C A A 1-K C C C C 1-L C C C A 1-MC C A A 1-N C B A C 1-O B B A C 1-P B B A C 1-Q B B A C 1-R B B A B 1-SC C A B 1-T B B A C 1-U A A A C

From the examples which are shown in Table 5 and 6, steels No. 1-1 to1-33 which satisfy the conditions of the first embodiment have favorablehot manufacturability, corrosion resistance, and impact characteristics.On the other hand, the steels No. 1-A to 1-U which do not satisfy theconditions of the first embodiment were inferior in all of hotmanufacturability, corrosion resistance, and impact characteristics.

As seen from the above examples, it is clear that it is possible toobtain an inexpensive alloy-saving type duplex stainless steel withfavorable hot manufacturability where the corrosion resistance isimproved by the addition of Sn according to the first embodiment.

Example 2

Description will be given of examples of the general-purpose duplexstainless steel below. The chemical compositions of the test steels areshown in Tables 7 to 10. Here, the remainder other than the componentswhich are described in Tables 7 to 10 is Fe and inevitable impurityelements. In addition, for the components which are shown in Tables 7 to10, portions where the amounts are not described show the impuritylevels. REM indicates lanthanoid rare earth elements and the amount ofREM shows the total of these elements. The numbers which are underlinedin the table indicate values outside of the ranges which are defined inthe second embodiment.

TABLE 7 Steel No. C Si Mn P S Cr Ni Co N Al Ca Cu Sn Other O 2-1Invention Examples 0.015 0.39 2.45 0.022 0.0005 26.5 4.48 0.254 0.0150.0021 1.43 0.07 0.0038 2-2 0.012 0.35 3.25 0.021 0.0007 27.3 4.83 0.650.235 0.023 0.0016 1.52 0.08 0.0032 2-3 0.021 0.42 3.45 0.023 0.000425.3 4.05 0.12 0.253 0.018 0.0018 1.03 0.05 Mo: 1.23 0.0034 2-4 0.0240.22 3.65 0.023 0.0005 23.5 2.35 0.32 0.245 0.025 0.0023 1.53 0.13 Mo:1.75 0.0028 2-5 0.023 0.53 1.52 0.024 0.0002 26.4 4.52 0.01 0.265 0.0030.0021 0.52 0.14 W: 0.35 0.0038 2-6 0.016 0.65 2.43 0.025 0.0006 25.13.85 0.23 0.245 0.016 0.0015 1.53 0.06 Mo: 1.25, 0.0042 W: 0.24 2-70.007 0.24 0.25 0.021 0.0005 26.5 4.03 0.53 0.246 0.012 0.0016 1.45 0.07V: 0.12 0.0043 2-8 0.026 0.74 3.35 0.023 0.0006 26.5 4.53 0.24 0.2240.017 0.0017 1.23 0.08 Nb: 0.034 0.0038 2-9 0.015 0.44 2.56 0.031 0.000526.4 4.52 0.21 0.236 0.021 0.0023 1.52 0.13 Ti: 0.007 0.0032 2-10 0.0140.42 2.75 0.033 0.0005 26.6 4.51 0.23 0.245 0.022 0.0021 1.48 0.09 V:0.07, 0.0037 Nb: 0.024 2-11 0.023 0.39 3.21 0.015 0.0004 26.4 4.36 0.850.234 0.028 0.0013 1.03 0.10 Nb: 0.047, 0.0028 Ti: 0.011 2-12 0.022 0.362.35 0.034 0.0006 26.3 4.42 0.03 0.253 0.013 0.0024 0.95 0.08 V: 0.13,0.0034 Nb: 0.015, Ti: 0.005 2-13 0.025 0.35 2.64 0.026 0.0005 23.8 3.850.15 0.238 0.024 0.0023 1.05 0.12 Mo: 1.52, 0.0040 V: 0.12 2-14 0.0180.31 2.48 0.024 0.0009 25.6 4.15 0.19 0.247 0.018 0.0019 1.12 0.08 Mo:0.52, 0.0042 V: 0.07, Nb: 0.034 2-15 0.019 0.28 2.54 0.026 0.0007 26.44.62 0.06 0.265 0.023 0.0022 1.33 0.06 B: 0.0023 0.0044 2-16 0.013 0.332.53 0.024 0.0006 26.6 4.58 0.14 0.267 0.021 0.0024 1.22 0.07 Mg: 0.00120.0031

TABLE 8 Steel No. C Si Mn P S Cr Ni Co N Al Ca Cu Sn Other O 2-17Invention Examples 0.024 0.37 2.54 0.025 0.0005 26.4 4.05 0.51 0.2580.028 0.0023 1.45 0.06 REM: 0.035 0.0034 2-18 0.025 0.45 2.56 0.0230.0005 26.5 4.45 0.25 0.265 0.0021 1.03 0.07 B: 0.0026, 0.0048 Mg:0.0007 2-19 0.027 0.51 2.51 0.025 0.0005 24.8 4.01 0.15 0.244 0.0160.0025 1.49 0.05 Mo: 1.23, 0.0036 V: 0.12, B: 0.0031, Mg: 0.0005 2-200.022 0.23 2.58 0.024 0.0005 23.3 3.52 0.36 0.228 0.026 0.0017 0.99 0.07Mo: 1.36, 0.0035 W: 0.75, V: 0.06, Ti: 0.004, B: 0.0026 2-21 0.011 0.262.48 0.023 0.0004 25.0 4.49 0.13 0.240 0.018 0.0022 1.05 0.06 Mo: 1.22,0.0034 V: 0.13, Nb: 0.045, Ti: 0.004, B: 0.0024, Mg: 0.0001 2-22 0.0160.12 2.36 0.025 0.0006 25.0 4.00 0.12 0.242 0.024 0.0023 1.48 0.02 Mo:1.35, 0.0035 V: 0.12, Nb: 0.015, Ti: 0.006, B: 0.0023, Mg: 0.0003

TABLE 9 Steel No. C Si Mn P S Cr Ni Co N Al Ca Cu Sn Other O 2-23Invention Examples 0.024 0.46 2.44 0.026 0.0005 25.3 4.23 0.16 0.2480.023 0.0025 1.46 0.05 Mo: 1.02, 0.0042 W: 0.32, V: 0.10, Nb: 0.021, Ti:0.005, B: 0.0024, Mg: 0.002 2-A Comparative Examples 0.016 0.37 2.130.022 0.0006 25.6 3.25 0.246 0.023 0.0008 2.85 0.12 0.0042 2-B 0.0130.41 2.65 0.027 0.0008 25.4 3.24 0.05 0.273 0.021 0.0014 2.23 0.240.0035 2-C 0.015 0.40 3.01 0.025 0.0006 25.1 4.00 0.10 0.251 0.0160.0021 0.05 0.10 0.0037 2-D 0.014 0.40 2.99 0.025 0.0006 25.0 4.02 0.100.249 0.017 0.0020 0.50 0.0036 2-E 0.036 0.39 2.98 0.024 0.0005 24.83.98 0.10 0.248 0.026 0.0021 0.47 0.04 0.0033 2-F 0.015 1.26 3.02 0.0240.0007 26.8 3.88 0.10 0.233 0.026 0.0017 0.49 0.06 0.0028 2-G 0.014 0.425.12 0.025 0.0005 25.1 3.87 0.11 0.256 0.020 0.0020 0.48 0.05 0.0043 2-H0.016 0.41 2.97 0.062 0.0005 26.5 3.76 0.09 0.232 0.014 0.0015 0.48 0.080.0040 2-I 0.016 0.43 2.98 0.024 0.0013 26.3 4.04 0.12 0.255 0.0130.0015 0.52 0.07 0.0041 2-J 0.012 0.45 2.42 0.026 0.0006 29.1 4.53 0.080.262 0.018 0.0020 0.53 0.09 0.0051 2-K 0.013 0.39 2.89 0.024 0.000724.9 1.78 0.35 0.249 0.019 0.0018 0.55 0.06 0.0038 2-L 0.016 0.42 2.510.025 0.0008 24.9 3.98 1.35 0.244 0.018 0.0023 1.02 0.08 0.0045 2-M0.014 0.38 2.47 0.025 0.0006 25.0 3.42 0.06 0.321 0.015 0.0016 0.49 0.050.0052 2-N 0.015 0.42 2.42 0.023 0.0007 24.8 4.02 0.07 0.253 0.0620.0022 0.52 0.06 0.0042 2-O 0.015 0.39 2.52 0.024 0.0007 24.9 4.01 0.090.245 0.008 0.0012 0.50 0.06 0.0053 2-P 0.014 0.40 2.46 0.023 0.000625.0 3.99 0.10 0.246 0.007 0.0021 3.53 0.11 0.0044

TABLE 10 Steel No. C Si Mn P S Cr Ni Co N Al Ca Cu Sn Other O 2-QComparative Examples 0.015 0.40 2.50 0.025 0.0007 25.0 4.00 0.09 0.2510.021 0.0021 0.04 Mo: 1.02 0.0038 2-R 0.014 0.39 2.48 0.026 0.0006 24.84.02 0.12 0.246 0.019 0.0017 0.03 0.02 Mo: 0.52, 0.0043 V: 0.06, B:0.0021 2-S 0.016 0.41 2.52 0.025 0.0005 25.1 2.35 0.01 0.265 0.0180.0023 1.83 0.16 Mo: 0.48, 0.0033 W: 0.12, Nb: 0.012, Ti: 0.006, B:0.0023 2-T 0.014 0.42 2.49 0.026 0.0006 25.1 4.03 0.03 0.262 0.0230.0048 1.02 0.07 Mo: 0.32 0.0032 2-U 0.013 0.48 1.65 0.024 0.0006 22.55.83 0.178 0.013 0.0023 0.05 Mo: 3.03 0.0035

Under the same conditions as Example 1, the manufacturing of the caststeel, the evaluation of the fracture reduction of area of the caststeel, the manufacturing of the hot-rolled material, the performing ofthe hot rolling with respect to the hot-rolled material, and theevaluation of the edge cracking were performed. The obtained evaluationresults are given in Tables 11 and 12.

Furthermore, the steel plates were subjected to a solutionizing heattreatment in the following manner. The steel plate was inserted into aheat treatment furnace at 1050° C. and heated for approximately 5minutes. Next, the steel plate was taken out, and then was subjected towater cooling to room temperature.

The corrosion resistance of the steel plate was evaluated by thecorrosion rate in the sulfuric acid.

The corrosion rate in the sulfuric acid was measured as follows. Testpieces of 3 mm thick×25 mm wide×25 mm long, were subjected to animmersion test for 6 hours in sulfuric acid including 2000 ppm of Clions, where the concentration was 15% and the temperature was 40%. Theweight before and after immersion was measured, and the rate of decreasein weight was calculated. Steels where the corrosion rate in thesulfuric acid was in a range of less than 0.1 g/m² per hour wereevaluated as A (good), steels where the corrosion rate in the sulfuricacid was in a range of 0.1 to 0.3 g/m² per hour were evaluated as B(fair), steels where the corrosion rate in the sulfuric acid was in arange of more than 0.3 g/m² per hour were evaluated as C (bad), and theevaluation results are given in Tables 11 and 12.

Under the same conditions as Example 1, the impact characteristics weremeasured. The obtained evaluation results are given in Tables 11 and 12.

TABLE 11 Edge Sulfuric Cracking Acid Impact Reduction ResistanceResistance Characteristics Steel Ni + of Area of of Steel of Steel ofSteel No. Co Ca/O PI Cast Steel Material Material Material InventionExamples 2-1 4.48 0.55 30.6 A A A A 2-2 5.48 0.50 31.1 A A A A 2-3 4.170.53 33.4 A A A A 2-4 2.67 0.82 33.2 A A A A 2-5 4.53 0.55 30.6 A A A A2-6 4.08 0.36 33.1 A A A A 2-7 4.56 0.37 30.4 A A A A 2-8 4.77 0.45 30.1A A A A 2-9 4.73 0.72 30.2 A A A A 2-10 4.74 0.57 30.5 A A A A 2-11 5.210.46 30.1 A A A A 2-12 4.45 0.71 30.3 A A A A 2-13 4.00 0.58 32.6 A A AA 2-14 4.34 0.45 31.3 A A A A 2-15 4.68 0.50 30.6 A A A A 2-16 4.72 0.7730.9 A A A A 2-17 4.56 0.68 30.5 A A A A 2-18 4.70 0.44 30.7 A A A A2-19 4.16 0.69 32.8 A A A A 2-20 3.88 0.49 31.4 A A A A 2-21 4.62 0.6532.9 A A A A 2-22 4.12 0.66 33.3 A A A A 2-23 4.39 0.60 32.6 A A A A

TABLE 12 Edge Sulfuric Cracking Acid Impact Reduction ResistanceResistance Characteristics Steel Ni + of Area of of Steel of Steel ofSteel No. Co Ca/O PI Cast Steel Material Material Material ComparativeExamples 2-A 3.25 0.19 29.5 C C A A 2-B 3.29 0.40 29.8 C C A A 2-C 4.100.57 29.1 A A C A 2-D 4.12 0.56 29.0 A A C A 2-E 4.08 0.64 28.8 A A B B2-F 3.98 0.61 30.5 A A A C 2-G 3.98 0.47 29.2 A A C A 2-H 3.85 0.38 30.2A A B B 2-I 4.16 0.37 30.4 C C B A 2-J 4.61 0.39 33.3 A A A C 2-K 2.130.47 28.9 C C B C 2-L 5.33 0.51 28.8 A A A A 2-M 3.48 0.31 30.1 C C A B2-N 4.09 0.52 28.8 A A A C 2-O 4.10 0.23 28.8 C C A B 2-P 4.09 0.48 28.9C C A B 2-Q 4.09 0.55 32.4 A A C A 2-R 4.14 0.40 30.5 A A C A 2-S 2.360.70 30.9 C C A A 2-T 4.06 1.50 30.3 B B B B 2-U 5.83 0.66 35.4 A A A A

From the examples which are shown in Table 11 and 12, thegeneral-purpose duplex stainless steels No. 2-1 to 2-23 which satisfythe conditions of the second embodiment have favorable hotmanufacturability, corrosion resistance, and impact characteristics. Onthe other hand, steels No. 2-A to 2-K and 2-M to 2-T which do notsatisfy the conditions of the second embodiment were inferior in hotmanufacturability, corrosion resistance, and impact characteristics. Inaddition, comparative example 2-L satisfied the characteristics;however, since a large amount of Co was contained, comparative example2-L was inferior in terms of cost. In addition, comparative example 2-Uis S31803 steel and is favorable in all of hot manufacturability,corrosion resistance, and manufacturability. However, the amounts of Niand Mo are high and comparative example 2-U is inferior in terms of costfor the purpose of the second embodiment.

As seen from the above examples, it is clear that it is possible toobtain an inexpensive general-purpose duplex stainless steel withfavorable hot manufacturability where the corrosion resistance isimproved due to the addition of Sn and Cu according to the secondembodiment.

INDUSTRIAL APPLICABILITY

According to the first and second embodiments, it is possible to providean alloy-saving type duplex stainless steel and a general-purpose duplexstainless steel which are inexpensive and where the corrosion resistanceis improved. These duplex stainless steel materials make an extremelysignificant contribution to industries because it is possible to use theduplex stainless steel materials in seawater desalination unit, tanksfor a transport ship, various types of containers, or the like.

1-4. (canceled)
 5. A duplex stainless steel comprising, in mass %: C:0.03% or less; Si: 0.05% to 1.0%; Mn: 0.1% to 4.0%; P: 0.05% or less; S:0.0001% to 0.0010%; Cr: 23.0% to 28.0%; Ni: 2.0% to 6.0%; Co: 0% to1.0%; Cu: 0.2% to 3.0%; Sn: 0.01% to 0.2%; N: 0.20% to 0.30%; Al: 0.05%or less; and Ca: 0.0010% to 0.0040%, with the remainder being Fe andinevitable impurities, wherein Ni+Co is in a range of 2.5% or more and aratio Ca/O of the amounts of Ca and O is in a range of 0.3 to 1.0, andPI shown by formula (1) is in a range of 30 or more to less than 40,PI=Cr+3.3Mo+16N  (1), (the chemical symbols in the formula (1) indicatethe amounts of the elements).
 6. The duplex stainless steel according toclaim 5, further comprising: either one or both of Mo: 2.0% or less, andW: 1.0% or less.
 7. The duplex stainless steel according to claim 5 or6, further comprising: one or more selected from V: 0.05% to 0.5%, Nb:0.01% to 0.15%, and Ti: 0.003% to 0.05%.
 8. The duplex stainless steelaccording to any one of claims 5 to 7, further comprising: one or moreselected from B: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% orless. 9-10. (canceled)